Before we talk about eagles, let’s take a moment to talk about trolls. They’re ugly. They regenerate. Everybody and their grandma knows that you’re supposed to finish them off with fire. This makes Mr. Troll the poster child for “adventure precedent.” All but the most inexperience players know to light the torches when a troll comes calling, and you can’t blame them either. This is the sort of metagaming that you have to include. I mean, what are the players supposed to do, stand around and pretend that they don’t know this dirt common piece of lore while the troll keeps getting up and wailing on them? Rather than being the kind of GM that goes, “No guys. You wouldn’t know to use fire,” I suggest that this is precisely where you want to try subverting expectations.

Imagine for a moment that you’re the GM. You want to challenge the players, but Mr. Troll is a solved encounter before he even hits the table. Why not invent a troll subspecies with a weakness to cold or sonic damage? You might go with one of the classics and apply the half-dragon template, making the troll immune to fire. I wound up doing something similar in my own game, giving some armor of fire/acid resistance to a troll boss. It was a proud moment watching the table turn immediately to problem-solving mode, using dimension door to teleport the armor off the big bad. While questionably legal, this is 100% the kind of thinking you want to encourage in your players. Creative solutions are fun, and if they work it makes players feel like a million bucks. Using known quantities like regenerating trolls, but twisting the circumstances away from the commonplace, is a great way to make these moments happen.

As for Prof. Tolkien’s eagles, the same sort of thinking applies. The world is a strange place, and might not resemble the familiar stereotypes from literature. You might run across evil unicorns, good undead, or a dragon in need of rescue from a wicked princess. This is not about breaking the rules of the game world, but about keeping your players on their toes. Just make sure not to subvert expectations too often, or you risk losing the surprise. Every once in a while though? Sure. Go ahead. Turn those kindly eagles into rapacious rocs. Chances are your players don’t have Knowledge (ornithology) anyway.